Support caching arbitrary container registries

[ajaffie]

What is the problem you're trying to solve
I'm trying to setup cache rules for all of my dependencies. Only one is currently unsupported, which is oci.external-secrets.io.

Describe the solution you'd like
It would be nice if a cache rule could be setup for any arbitrary registry that can be used without authentication.
Support for oci.external-secrets.io is acceptable as well, considering it is probably fairly widely used.

Update: The images are also available at ghcr.io/external-secrets/external-secrets, which I discovered looking at their github releases.